Removal of sulfur compounds from wastewater

ABSTRACT

The invention relates to a process for removal of sulfur compounds from wastewater, which process provides recovery of elemental sulfur. The process is carried out in two separate anaerobic reactors. In the first reactor organic compounds are converted to acid compounds and sulfur compounds are converted to sulfide compounds. The sulfide compounds are stripped from said first reactor with a stripping gas. Subsequently, the effluent of the first reactor is fed to a second anaerobic reactor, in which organic compounds, such as the acid compounds, are converted to produce biogas, while any unconverted sulfur compounds from the previous step are converted to sulfide compounds as well. Finally, the sulfide compounds are removed from the second reactor in a stripping system, and converted to elemental sulfur in an adsorber.

[0001] The invention relates to a process for anaerobic removal ofsulfur compounds from wastewater.

[0002] Anaerobic biological processes for the treatment of wastewaterhaving a high COD (chemical oxygen demand) value are known in the art.In such known processes, organic compounds are converted to biogas (agas mixture comprising CO₂ and CH₄) at temperatures above about 25° C.When wastewater contains high amounts of sulfur compounds (e.g., morethan ca. 100 mg S/dm³), such as sulfates, problems may occur inoperating such anaerobic reactors due to the presence of sulfide, whichis formed under anaerobic conditions from sulfate. Sulfide may inhibitthe methane formation. Also sulfide will give rise to H₂S formation atpH<9. H₂S is a toxic and corrosive gas and requires measures to controlodour. The sulfide may be reoxidized into sulfate, but this requires anadditional aerobic step. Furthermore, regulations often impose a stronglimitation on the amount of sulfur compounds that can be discarded intothe environment.

[0003] As a consequence, there is a need for processes to remove sulfidefrom wastewater. In the art different approaches have been suggested forsulfide removal processes. For example, EP-A-0 766 650 describes ananaerobic process in which H₂S is stripped from a methanogenic reactorusing a stripping gas. The H₂S rich stripping gas is fed to a scrubberwhere the H₂S is converted into elemental sulfur. The scrubber isoperated with a regenerable redox liquid that contains an iron(III)chelate or an iron(III) complex. During this absorption process sulfideis converted to elemental sulfur, while Fe(III) is reduced to Fe(II).Fe(II) is reoxidized to Fe(III) by air in a separate aerator. This knownprocess is particularly suited for the treatment of alkaline wastewater,such as tannery wastewater.

[0004] However, the known processes have a number of drawbacks.

[0005] The anaerobic reactor in known processes is operated at arelatively high pH (8-8.5), in particular when alkaline wastewater istreated having a pH of 9-12. As a result, the stripping of H₂S is slow,since H₂S dissolves more easily in water at higher pH. To compensate forthis, equipment of a large volume has to be employed, in particular thestripper column, which has to be operated with large volumetric flows.An example of such a process employing an external stripper is found inU.S. Pat. No. 5,500,123. Alternatively, acid, such as formic acid, canbe added. Both alternatives bring about high equipment and/or operatingcosts. Another result of operating the anaerobic reactor at a relativelyhigh pH is that carbonate may precipitate in the stripper, which leadsto fouling and clogging of the equipment.

[0006] When the H₂S loaded gas is scrubbed in the known processes,inevitably an amount of CO₂, which is present in the stripped gas aswell, is absorbed in the scrubber liquid. This CO₂ is eventually ventedwhen the scrubber liquid is regenerated. This net removal of CO₂ fromthe system gives rise to a further increase of the pH. Moreover, theformation of carbonate salts due to the presence of CO₂ in the stripperequipment may give rise to fouling and clogging. The precipitation ofcarbonate salts may be prevented by lowering the pH, however, thislowers the rate of absorption and reaction of H₂S, as a result of whichlarger absorbers and volumetric flowrates are required.

[0007] These drawbacks become apparent for example in the process ofEP-A-0 766 650, where the aqueous effluent needs to be recirculated overthe stripper a number of times in order to lower the sulfideconcentration, since per pass only a small amount of sulfide isstripped. This is the result of the unfavourable equilibrium at higherpH values. This recirculation results in further disadvantages, sincethe anaerobic reactor is operated at a higher hydraulic loading rate,which may lead to rinsing out of methanogenic sludge from the reactor.

[0008] Yet another disadvantage of known processes is that the elementalsulfur that is formed does not accumulate exclusively in the sulfursettling tank, but also forms deposits on walls of the tubing, vessels,sprayers, blowers, pumps, packing material, etc., which causes the needfor regular cleaning of the equipment.

[0009] It is an object of the present invention to provide a processwhich obviates, at least partly, some or all of the above mentionedproblems. The process of the invention thus aims to provide animprovement to anaerobic wastewater treatment with elemental sulfurrecovery.

[0010] In accordance with the present invention, there is provided aprocess for removal of sulfur compounds from wastewater, comprising thesteps of: a) converting in a first anaerobic suspended sludge reactororganic compounds present in the wastewater to acid compounds, formingan effluent comprising the acid compounds; b) converting the sulfurcompounds to sulfide compounds in the first reactor, the remainder ofthe sulfur compounds being also comprised by said effluent; c) removingsulfide compounds in said first reactor with a stripping gas, forming agaseous effluent; d) converting the acid compounds in said effluent tobiogas in a second anaerobic reactor; and e) converting sulfur compoundsin said effluent to sulfide compounds in the second anaerobic reactor,forming a second liquid effluent.

[0011] It will be understood that by converting the compounds in stepsa), b), d) and e), and by stripping in step c) is meant that at least anessential part of the respective compounds present are converted,stripped or removed, the non-converted, non-stripped and non-removedcompounds leaving the reactor in the effluent.

[0012] The organic compounds present in the wastewater are converted toacid compounds, in particular organic (carboxylic) acids, such as(lower) fatty acids. The conversion of organic compounds to acidcompounds, as well as the conversion of sulfur compounds to sulfide isestablished using microorganisms known to the skilled person. Foracidification of organic compounds bacterial species from the generaClostridium, Ruminococcus, Propionibacterium, Selenomonas,Micrormonospora, the family of the Lactobacteriaceae, and thermophilicclostridia can be used, as well as anaerobic sludge in which they arepresent. For the production of sulfide from sulfur compounds bacterialspecies from the genera Desulfouibrio, Desulfotomaculum, Desulfobacter,Desulfococcus, Desulfuromonas, Desulfonema, Desulfobulbus and forthermophilic applications Sulfolobus can be used, as well as sulfatereducing sludge or anaerobic sludge in which they are present.

[0013] The first (acidification) reactor typically works at 10-40° C.(mesophilic range) or 40-80° C. (thermophilic range), preferably 30-35°C., pressures between 0.3 and 4 bara, mostly 1 bara and biomassconcentrations between 0.1 and 10 g dry weight/dm³, mostly 3 g/dm³. Theratio between stripping gas flow rate and wastewater flow rate can be 3to 100, but is typically 15.

[0014] The term ‘suspended sludge reactor’ is used in the presentdescription and claims in its ordinary meaning and encompasses, as theskilled person knows, reactors in which the sludge is essentially freeto move through the reactor, viz. reactors in which the sludge isessentially not bound to a static surface in the reactor. Such reactorsdo therefore not rely on measures to increase the surface area which mayserve as a substrate for the microorganisms, such as packings, etc. Thesludge in these type of reactors is kept in suspension by means ofagitation, e.g. by mixing using an agitator, by recirculating theliquids or by bubbling gas through the liquids.

[0015] As a result of the production of acid compounds, the pH in thefirst reactor will be relatively low. The pH in the first reactor willdepend on the type of wastewater to be treated, but is generallymaintained at a value lower than 9, preferably lower than 8, mostpreferably between 6 and 7.5. Because of the low pH, the sulfide formedin the first reactor is relatively easily stripped. Moreover, only arelatively small amount of CO₂ is formed. A substantial amount,typically about 50% or more, of the sulfur compounds is converted tosulfide, and 50-100% of the produced sulfide can be absorbed in anabsorber. The non-absorbed sulfide is passed to the second reactor, aswell as other sulfur compounds, if present.

[0016] Apart from acid compounds, the effluent from the first reactormay comprise other organic compounds, such as organic compounds thathave not been converted in the first reactor or organic compounds thatare intermediates from the reactions in the first reactor. Essentiallyall types of organic compounds may be converted to biogas in the secondanaerobic (methanogenic) reactor. The conversion to biogas, as well asthe conversion of remaining sulfur compounds in this second reactor isestablished using microorganisms known to the skilled person. Thebacterial species that are typically employed for conversion of sulfurcompounds were mentioned above as typical species for converting sulfurin the first reactor. For methane production bacterial species from thegenera Methanococcus, Methanobacterium, Methanocarcina, Methanothrix,Methanophanus and Methanobrevibacter can be used. For thermophilicapplications Methanobacterium therinoautotrophicum or species fromMethanothermus can be used, as well as anaerobic sludge in which theyare present The second (methanogenic) reactor typically works at 10-40°C. (mesophilic range) or 40-80° C. (thermophilic range), preferably30-35° C., pressures between 0.3 and 4 bara, mostly 1 bara and biomassconcentrations between 1 and 50 g dry weight/dm³, mostly 30 g/dm³. Theratio between stripping gas flow rate and wastewater flow rate can be 3to 100, but is typically 20.

[0017] The sulfide loaded stripping gas leaves the first reactor and isprocessed in an absorber unit. A suitable absorber unit is described inEP-A-0 766 650 as mentioned herein-above The parts of this publicationwhich deal with absorption of H₂S are incorporated herein by reference.In detail, H₂S is absorbed from a gaseous stream by contacting the gaswith a regenerable redox liquor, having a pH from about 4 to 7.Preferably, the liquor comprises a transition metal complex, such as anFe(III) complex, or chelated iron, such as ethylenediamine tetra-aceticacid (EDTA). The metal complex is preferably used in a concentration ofabout 0.01 to about 0.1 M.

[0018] Alternatively, the absorption of sulfide may be carried out withan alkaline scrubber, in which the gas is contacted with an alkalinesolution and the H₂S is converted to alkalihydrosulfide. Thehydrosulfide may be converted to alkalisulfide (e.g. Na₂S), which may bereused in e.g. a tannery process. This type of H₂S absorber isparticularly preferred for tannery wastewater treatment processesaccording to the invention, since Na₂S is used as a raw material intanneries.

[0019] The low CO₂ production in the first reactor is very favourable.Since only little CO₂ is produced, the sulfide loaded stripping gas willbe relatively low in CO₂ content. As a result, the sulfide may beremoved from the gas, using means such as a conventional alkalinescrubbing process. If CO₂ were to be produced in substantial amounts,this would be removed by the alkaline solution as well. This wouldresult in high alkali consumption in the absorber and in a net removalof CO₂ from the first reactor, leading to increased pH values in thisreactor. For this reason, the low production of CO₂ is a considerableadvantage of the present invention.

[0020] Preferably the process of the invention further comprises a stepf) for removing the sulfide compounds from the second effluent (i.e. theliquid effluent from the second reactor) with a stripping system. Again,by removing in step f) is meant that at least an essential part of thesulfide present is removed, the non-removed sulfide leaving the reactorin the effluent. By removing a substantial amount of sulfide in thefirst reactor, removal of sulfur compounds in the form of sulfide can becarried out more easily in the second reactor, since the concentrationof sulfide in the effluent of the first reactor is already significantlylower. Typically, about 50% of the sulfur present in the wastewater isremoved. Moreover, the methanogenesis process in the second reactor willbe less influenced by the presence of sulfide. The use of two separateanaerobic reactors is thus advantageous, since it is not required toreturn desulfidized water to the methanogenic reactor to decreasesulfide concentration. This prevents (1) a too high hydraulic loadingrate in this reactor and as a consequence (2) risk of sludge wash out,as well as (3) lower methanogenic activities. This is in particularimportant for wastewater types which do not lead to granular sludgeformation in methanogenic reactors, but to the formation of flocculantsludge, e.g. tannery wastewater.

[0021] Another important aspect of the present invention is that thesulfide is removed from the first reactor by feeding a stripping gasthrough the liquid in the reactor, e.g. by bubbling. This makes the useof an external stripper superfluous, thus saving on installation andoperation costs. Also, by using a stripping gas, no H₂S will accumulatein the headspace of the first reactor, which would occur when the liquidwas fed to an external stripper.

[0022] In this respect, mention is made of U.S. Pat. No. 4,614,588,which describes a method for reducing the sulfur content of wastewaterby employing two reactors. Sulfide is removed by exhausting the gas fromthe first reactor. To obtain sufficient separation of H₂S gas (i.e. toproduce sufficient H₂S pressure in the head space of the reactor), it isrequired in the process of U.S. Pat. No. 4,614,588 that the pH is keptbelow 6. Such a requirement poses a strong limitation on the versatilityof the removal process, since alkaline wastewater, e.g. tannerywastewater, cannot suitably be treated in such a process, since it wouldlead to a pH of higher than 6.

[0023] Another advantage of flowing stripping gas through the firstreactor is that it enables a good mixing of the water, the anaerobicsludge flocs and the gas, whereby the sludge may be suspended in thewater by the gas. This enables the use of the anaerobic contact process(ACP). This feature is specifically constitutes an advantage over thedisclosure of U.S. Pat. No. 4,735,723, which describes a process for theremoval of sulfur compounds from wastewater using two anaerobicreactors. According to this document, a fluidized bed reactor ispreferred to carry out the acidification reaction, preferably combinedwith an external stripper. In fluidized bed reactors a high sludge ageis employed. As a result of the high sludge age, microorganisms willdevelop that will convert organic acids to methane. As a result, the pHof the first reactor will increase, which is undesirable as was set outabove. Sludge ages that are typical for fluidized bed reactors aregenerally too high for the process of the present invention. The reactorsystems suggested in U.S. Pat. No. 4,735,723 separate all water andsludge in the reactor, as a result of which the sludge remains in thereactor for a period of time that is generally too high for the processof the present invention.

[0024] According to the present invention it is essential that theaverage sludge age is kept low, preferably below 5 days, more preferablyfrom 1-4 days, most preferably at about 3 days.

[0025] This can be obtained by carrying out the separation of water andsludge in an external separator. This can be obtained by using the ACPprocess, which forms a preferred embodiment of the invention, especiallyfor the treatment of alkaline wastewater.

[0026] In the ACP process wastewater is mixed with recycled sludgesolids and then converted in a reactor sealed off from the entry of air.The contents of the reactor are completely mixed and the sludge ispresent in the form of suspended flocs. The flocs are kept in suspensionby agitation. In the reactor the bacteria in the flocs biologicallyconvert the compounds in the wastewater to other compounds. After thereaction, the mixed liquor, containing sludge flocs, is transported to asettling tank in which the flocs settle and the relatively clearsupernatant is the effluent. At the bottom section of the settler a moreconcentrated suspension of flocs is formed. A large part of thissuspension is recycled to the reactor, a smaller part is discharged. Bysettling and recycling, the formation of flocs by biomass in the systemis promoted (by selection). Because of the completely mixedcharacteristics of the system, the sludge retention time (sludge age)can be controlled precisely by the discharge flow rate of the settledsludge. For example, if every day one-third of the total mass of sludgeis removed from the system, the sludge age will be 3 days. Sinceacetoclastic methanogens (which convert acetic acid into methane) needsludge ages of more than 5 days (because of their high doubling time),they will not grow in a system with a sludge age of 3 days.

[0027] The ACP process is typically used for wastewater with CODconcentrations of 1500-5000 mg/l and the hydraulic retention time istypically 2-10 hours. Although problems have been reported withseparation of sludge and gas in methanogenic anaerobic contactprocesses, in the present non-methanogenic system a better separation isobtained, as the gas is not produced in the flocs but added in form ofbubbles in the water phase.

[0028] Since the methanogenesis from fatty acids is minimized using theACP process, an optimal accumulation of fatty acids and a correspondinglow pH is obtained.

[0029] Other types of reactors known in the art are less or not suitableto obtain a suitable sludge age. For example, packed bed reactors, suchas anaerobic filter or fixed film reactors, do not enable the control ofsludge age, since in these reactors the biomass (from which the sludgeis formed) is fixed to the packing.

[0030] In this respect, reference is made to a publication by E. Särner(Wat. Sci. Tech., 22 (1990) 395-404), which describes a sulphur removalprocess that uses an anaerobic trickling filter (Antric filter as apretreatment step. The pH in this step is kept low by applying arecirculation stream over the filter. The accumulation of organic acidsleads to a pH lower than 6, the minimum pH for methanogenesis. However,such low pH cannot be guaranteed in case alkaline wastewater is treated.Than the high pH and the high sludge age will lead to conversion oforganic acids into methane. In addition, even under low pH conditions,methanogenic activity can occur in the Antric filter; according toSärner, the choice for biofilm processes results in considerable CH₄formation, since it is inevitable that at different biofilm depths,different environmental conditions are found, among which conditionsthat give rise to breakdown of organic acids, whereby CH₄ is produced.In addition, breakdown of organic acids results in a rise of pH, whichmakes this process unsuitable for treatment of alkaline waste water.

[0031] Also fluidized bed reactors are less suitable or even unsuitable,since these type of reactors are characterized by a considerable spreadin residence time of the biomass. When fluidized support particles, suchas sand, need to be partly refreshed, a portion of support particlescovered with sludge will remain in the reactor. As the biomass growth onthe fresh sand particles will be much slower than growth on theparticles already covered with biomass, a large spread in sludge agewill develop. As a result a considerable amount of biomass with a veryhigh sludge age will be present in the reactor, which makes thedevelopment of acetoclastic methanogens possible, which results in theconsumption of acetic acid.

[0032] Carrying out the desulfurization process in two separate reactorsaccording to the invention, does not affect significantly theinstallation costs of the total plant.

[0033] By carrying out the process in accordance with the invention, theproblem of precipitation of carbonate salts in the stripper as used inknown processes is overcome. As a result of the lower pH, a lowerflowrate of the stripping gas can be employed as well as a lowergas/liquid exchanging surface, making the need for an external (packed)stripper, operated with gas as the continuous phase unnecessary.

[0034] The biogas that is produced in the second reactor, will containsulfide as well, which has to be removed. This can be done byconventional means, such as by the method described in EP-A-0 766 650,which was described herein-above as a means to remove the sulfide fromthe product gas of the first reactor. By removing the sulfide from theproduct gas, clean biogas is obtained, which can be used for a varietyof purposes, which will be apparent to the skilled person. The sulfideis converted in the absorber to elemental sulfur. The sulfur containingeffluent of the absorber is fed through conventional means, such as acoagulator, a flocculator and/or a settling tank to produce a sulfurslurry, which also can be used for a variety of purposes, which will beapparent to the skilled person.

[0035] The liquid effluent of the second reactor may still contain aconsiderable amount of sulfide. These sulfide compounds can be removedfrom the liquid effluent, using a stripping column, which is fed with astripping gas comprising CO₂. The stripping gas is brought into contactwith the liquid by any suitable means, such as spraying or trickling theliquid down and flowing the stripping gas in countercurrent. The sulfideloaded stripping gas is then fed to an absorber unit, which is similarto the one described above, to convert the sulfide to elemental sulfur.The remaining desulfidized gas is vented.

[0036] Stripping of H₂S from liquids proceeds more easily, i.e. athigher rates and/or more favorable equilibrium conditions, at relativelylow pH (pH about 6.5 to 7.5) and higher temperatures (about 30-40° C.,preferably at about 31-35, most preferably at about 33° C., instead ofabout 25° C.). For this reason it is preferred to use a combustionoff-gas as the stripping gas in stripping the H₂S from the liquideffluent of the second reactor. The CO₂ in the combustion off-gasprovides a lower pH to the liquid, which enhances H₂S stripping andprevents the formation of carbonate deposits in the stripper, whereasthe heat present in the off-gas provides a higher temperature for thestripping process.

[0037] The combustion gases can be an effluent from conventionalcombustion processes. Another advantage of this embodiment is that noextra measures have to be taken to obtain CO₂ externally. It ispreferred to use biogas produced with the process of the invention forproducing the off-gases, since it can be burned on-site to produce theseoff-gases.

[0038] In addition, it was surprisingly found that, despite the lowersolubility of H₂S in Fe(III)chelate containing liquors, the highertemperatures of the sulfide loaded stripping gas, which results in anincreased temperature in the absorber, the rate of H₂S absorption isenhanced in the absorption step. Also the flocculation of sulfur isenhanced by higher temperatures, as will be discussed below.

[0039] In a further preferred embodiment of the invention, the removingof the sulfide in step f) comprises contacting the liquid effluent fromthe second reactor with an oxygen containing gas, preferably air. Thusthe stripping of the liquid effluent from the second reactor is carriedout using a gas mixture, which further comprises oxygen. The oxygenconcentration will generally be up to 20 vol. %. Preferably the oxygenconcentration is from 5-20 vol. %, most preferably about 14 vol. %. Tothis end air can be mixed with the stripping gas, such as the off-gasmentioned above. In the process described in EP-A-0 766 650 thereduction of Fe(III) to Fe(II) by sulfide (by which sulfur is produced)and the reoxidation of Fe(II) is carried out by separate processes,i.e., in separate columns. According to EP-A-0 766 650 the oxygenconcentration has to be carefully controlled in order to preventintroduction of oxygen in the system. It is therefore surprising thataccording to the present process it is not required to operate theabsorption of sulfide in the absence of oxygen. The oxygen which isintroduced through the stripping column in the absorber can instead beused to regenerate the Fe(II) to Fe(III) (or another suitable redoxsystem that is used) in the absorber. This makes the presence of aregenerator no longer required in this embodiment. Also means to controlthe oxygen concentration are no longer required, adding to the economicadvantage of this embodiment.

[0040] A further preferred embodiment which uses oxygen to regeneratethe redox liquor, comprises feeding the sulfide from the gaseouseffluent from the second reactor and the sulfide from the liquideffluent from the second reactor to separate absorbers using the samerecirculating redox liquor, in which the sulfide from said gaseouseffluent from the second reactor is contacted with said redox liquor inco-current. The stream of biogas is essentially smaller than the gaseouseffluent of the stripper used to treat the liquid effluent of the secondreactor. Therefore the absorber treating the biogas can be smaller aswell. However, the same absorption liquid may be used, provided thattransfer of oxygen from freshly regenerated redox liquor to the biogasis prevented. This can be obtained by operating the absorber for thebiogas such that the biogas and the regenerated absorption liquid flowin co-current. The oxygen present in the absorption liquid thus canreact with the reduced metal (e.g. Fe(II) is oxidized to Fe(III)).

[0041] In another preferred embodiment, the liquid effluent of thesecond reactor is passed over a stripper only once. In conventionalprocesses the sulfide containing effluent of a methanogenic reactor hasto be recirculated over a stripper in order to obtain an acceptabledecrease in sulfide concentration. This is also caused by theunfavorable chemical equilibrium as a result of the high pH employed. Adisadvantage of recirculation of the aqueous effluent is that the higherhydraulic loading rate of the stripper requires a larger stripper. Thesecond prerequisite for a smaller stripper is that no recirculation ofstripper effluent over the methanogenic reactor is required, which bearsthe disadvantage of too high hydraulic loading rates and sludge washout. According to the present invention, however, recirculation is notrequired, since a substantial amount of sulfide is already removed inthe first reactor (up to about 50%), as a result of which, themethanogenesis in the second reactor is hardly adversely affected. Byusing a suitable stripping gas, such as CO₂ as described above, anoverall sulfur removal of more than 90% can be obtained with the processof the invention, using no recirculation in the stripper for the liquideffluent of the second reactor.

[0042] Another preferred embodiment of the process of the invention isany of the processes as described above, in which step f) comprisescontacting the sulfide containing biogas effluent and/or a gaseoussulfide containing effluent obtained from stripping the liquid effluentof the second reactor with a redox liquor at a pH of 7-9, preferably ata pH of about 8.5. In the art a pH of 4-7 is said to be optimal foroperating the absorption step with a redox liquor, in order to minimizeabsorption of CO₂ from the sulfide containing gas. If net removal CO₂from the system takes place, it is required to replenish it fromexternal sources in order to prevent a pH that would be too high foroperating the anaerobic reactor and the stripping process successfully.It was found, however, that the absorption of sulfide is enhanced when ahigher pH of the redox liquor is employed. When a pH of about 7-9,preferably of about 8.5 is used, both the rate of absorption of H₂S andthe rate of conversion to elemental sulfur are increased. Whenconventional phosphate buffers are used in this liquor, this will giverise to considerable CO₂ losses. However, when bicarbonate buffers areused, preferably in concentration of 0.5-1 M, CO₂ losses can beprevented even at relatively high gas phase CO₂ concentrations expected(about 10%).

[0043] In another preferred embodiment, step f) comprises contacting thesulfide containing biogas effluent and/or a gaseous sulfide containingeffluent obtained from stripping the liquid effluent of the secondreactor with a redox liquor, by which sulfide is converted to sulfur,forming a sulfur comprising effluent, which is fed to a coagulationtank, a flocculation tank and a settler tank, thus forming a sulfur richeffluent. It was found that deposition of solid sulfur on equipment,such as tubing, vessel, gas blowers, pumps and packing material, can beprevented by removing the sulfur as quickly as possible from the redoxliquor. To this end the redox liquid is first fed to the coagulationtank, in which it is stirred vigorously for about 1-3 minutes, givingrise to the formation of small sulfur crystals After the coagulationtank the liquid is fed to a flocculation tank in which the suspension isstirred mildly for 3-10 minutes, by which the crystals will form largerflocs. Flocculation can further be enhanced by the addition ofconventional flocculation agents. The flocs formed can be removed easilyby means of the settler tank to which the liquid is fed next. It wasfurther found that coagulation of sulfur crystals proceeds more rapidlyat elevated temperatures, which constitutes another advantage for theuse of off-gases from combustion processes.

[0044] The process of the invention is particularly useful in treatingwastewater with high COD concentrations (>2000 mg/dm³).

[0045]FIG. 1 describes a preferred embodiment of the process of theinvention. This process uses an alkaline scrubber to remove sulfide fromthe stripping gas in the first reactor. A wastewater influent is fed toa first (acidifying) reactor. By bubbling stripping gas through thefirst reactor the sulfide produced in the first reactor is stripped fromthe reactor. In the alkaline scrubber the stripping gas is desulfidizedby contacting it with an alkaline solution. This produces a streamcomprising hydrosulfide, which can be used for further processing andclean stripping gas which is recycled to the bottom of the firstreactor. To prevent underpressure in the stripping gas recycle of thefirst reactor (e.g. as a result of absorption of gases in the reactor)or overpressure in this reactor (e.g. as a result of methaneproduction), it may be useful to connect this stripping gas circuit withthe biogas effluent from the second reactor. The liquid effluent fromthe first reactor is passed to a settler tank, in which the sludgesettles and is recirculated to the reactor, while the liquid is fed tothe second (methanogenic) reactor. The liquid effluent of the secondreactor is sprinkled from the top of a stripper column, which is fed atthe bottom with a mixture of combustion off-gas and air. The liquideffluent of this stripping column, which is essentially desulfurized maybe discharged. The biogas produced in the second reactor is passed tothe top of an absorber (Absorber 1) in which it is contacted inco-current with fresh redox liquor. The purified biogas leaves Absorber1 at the bottom. Optionally it can be mixed with hydrogen gas. Thishydrogen gas can be produced by an electrochemical reactor, as isdescribed in EP-A-0 766 650. The stripping gas from the stripper whichis used to desulfidize the liquid effluent of the second reactor is fedto another absorber (Absorber 2). In Absorber 2 the sulfide loadedstripping gas is brought into contact with a fresh redox liquor toconvert sulfide to sulfur. Absorber 2 is operated in countercurrent. Thepurified gas leaving at the top of absorber 2 is vented. The redoxliquors in both absorbers are obtained from the same recirculatingsystem. The bottom products of the absorbers, containing elementalsulfur, are fed to a coagulation tank, a flocculation tank and to asulfur settling tank. The sulfur is obtained as a slurry from the bottomof the settling tank.

EXAMPLES Example 1

[0046] An anaerobic reactor of 5 l was fed with tannery wastewater witha pH of 10. The wastewater contained 3500 mg COD, 50 mg sulfide and 1350mg sulfate per liter, and was added at a flow rate of 40 l/day. Thesludge residence time was higher than the hydraulic residence time inthe reactor because of sludge sedimentation in the bottom part of thereactor. By daily removing ⅓ of the sludge present, the sludge age wasset at 3 days. In the top of the reactor gas was sparged, after leavingthe reactor the gas was bubbled through a solution of NaOH at a pH of 10and returned to the reactor sparger again. The gas flow rate was 100l/h.

[0047] The effluent of the reactor contained 3200 mg COD, 2000 mg VFA(volatile fatty acids), 150 mg sulfate and 30 mg sulfide per liter, andthe pH was 7.3. The H₂S concentration in the gas leaving the reactor was6400 ppm and after passing the absorber, the concentration was 3500 ppm.The CO₂ concentration in the gas was 1.5% and 1.3% after the absorber.

[0048] The results indicate that a large part of the organic compoundsare converted into volatile fatty acids and that because of a lowmethanogenic activity the acids were allowed to accumulate, thus leadingto a low pH and enhancing the stripping process. Simultaneously about90% of the sulfate was reduced to sulfide. This sulfide was removed for90% by stripping and absorption. The alkali scrubber relatively (%)removed more H₂S than CO₂.

Example 2

[0049] In a pilot plant H₂S rich gas was produced in a stripper bycontacting the gas with effluent from an anaerobic reactor. This gas waspassed through a 3 m³ absorber column with a height of 3.5 m,counter-current with a redox liquor flow of 5 m³/h. The redox liquor wastrickled over the plastic column packing material and containedFe(III)EDTA as the main reactant. Its pH was varied using NaOH and CO₂.Two different gas loading rates were tested (50 and 100 m³/h). Thetemperature was 30° C. The results are summarised in table 1. TABLE 1H₂S removal efficiencies in an absorber at various conditions Gas flowpH of H₂S in absorber H₂S H₂S rate redox influent gas in absorber eff-removal eff- (m³/h) liquor (ppm) luent gas (ppm) iciency (%) 50 7.325,000 600 97.6 50 7.7 30,000 440 98.5 50 8.0 10,000 175 98.3 100 7.510,000 775 92.3 100 7.9 9,000 400 95.6 100 8.0 7,000 300 95.7 100 8.314,000 425 97.0 100 8.4 9,000 300 96.7

[0050] A higher pH is beneficial to reach lower effluent H₂Sconcentrations and/or higher H₂S removal efficiencies. Because of thehigher gas residence times in the absorber column, higher removalefficiencies were obtained at lower gas flow rates.

1. Process for removal of sulfur compounds from wastewater, comprisingthe steps of: a) converting in a first anaerobic suspended sludgereactor organic compounds present in the wastewater to acid compounds,forming an effluent comprising the acid compounds; b) converting thesulfur compounds to sulfide compounds in the first reactor, theremainder of the sulfur compounds being also comprised by said effluent;c) removing sulfide compounds in said first reactor with a strippinggas, forming a gaseous effluent; d) converting the acid compounds insaid effluent to biogas in a second anaerobic reactor; and e) convertingsulfur compounds in said effluent to sulfide compounds in the secondanaerobic reactor, forming a second liquid effluent.
 2. Processaccording to claim 1, which further comprises a step f) of removing thesulfide compounds from said second effluent with a stripping system. 3.Process according to claim 1 or 2, in which the gaseous effluent of stepc) is contacted with a redox liquor or an alkaline solution, therebyremoving sulfide from the gaseous effluent and producing clean strippinggas which is recirculated to the process in step c).
 4. Processaccording to any of the previous claims, wherein the sludge age in thefirst reactor is from 1-4 days, preferably about 3 days.
 5. Processaccording to any of the previous claims, wherein an anaerobic contactprocess (ACP) is carried out in the first reactor.
 6. Process accordingto claim 2-5, wherein the removing of the sulfide in step f) comprisescontacting the liquid effluent from the second reactor with a strippinggas comprising CO₂, which is preferably an off-gas from a combustionprocess.
 7. Process according to claim 2-6, wherein the removing of thesulfide in step f) comprises contacting the liquid effluent from thesecond reactor with an oxygen containing gas, producing a sulfidecontaining gaseous effluent and feeding this gaseous effluent to anabsorber in which it is contacted with a redox liquor, which absorbs thesulfide.
 8. Process according to any of the previous claims, whichfurther comprises: g) contacting the biogas from step d) with a redoxliquor, which absorbs sulfide in the redox liquor; h) regenerating theredox liquor by contacting it with an oxygen containing gas.
 9. Processaccording to claim 7 or 8, wherein the sulfide from the biogas effluentfrom the second reactor and the sulfide stripped from the liquideffluent from the second reactor are absorbed in separate absorbersusing the same recirculating redox liquor, and wherein the sulfide fromsaid gaseous effluent from the second reactor is contacted with saidredox liquor in co-current.
 10. Process according to claim 2-9, whereinstep f) comprises stripping the liquid effluent from the second reactorwithout recycling the liquid.
 11. Process according to claim 2-10,wherein step f) comprises contacting the sulfide containing biogaseffluent and/or a gaseous sulfide containing effluent obtained fromstripping the liquid effluent of the second reactor with a redox liquorat a pH of 7-9, preferably at a pH of about 8.5, which redox liquorcomprises a bicarbonate buffer.
 12. Process according to claim 2-11,wherein step f) comprises contacting the sulfide containing biogaseffluent and/or a gaseous sulfide containing effluent obtained fromstripping the liquid effluent of the second reactor with a redox liquor,by which sulfide is converted to sulfur, forming a sulfur comprisingeffluent, which is fed to a coagulation tank, a flocculation tank and asettler tank, thus forming a sulfur rich effluent.